The present invention relates to the field of lubricants. More particularly, the present invention relates to a lubricant composition comprising at least one branched polyglycerol ether, and more particularly at least one branched polyglycerol ether and at least one dispersant. The lubricant composition according to the invention has good fuel economy properties. The lubricant composition according to the invention moreover has good thermal resistance properties.
The present invention also relates to a method utilizing this composition. The present invention also relates to a method for reducing the fuel consumption of a vehicle utilizing this lubricant composition. The present invention also relates to the use of a branched polyglycerol ether as a friction modifier in a lubricant composition.
The worldwide spread of the automobile since the end of the last century poses problems regarding global warming, pollution, the security and use of natural resources, in particular the depletion of oil reserves. Following the establishment of the Kyoto protocol, new standards protecting the environment require the automobile industry to construct vehicles having reduced pollutant emissions and fuel consumption. As a result, the engines of these vehicles are subject to increasingly stringent technical constraints: in particular they run more quickly, at increasingly high temperatures, and are required to consume less and less fuel.
The nature of engine lubricants for automobiles has an influence on the emission of pollutants and on fuel consumption. Engine lubricants for automobile engines, called energy-saving or “fuel-eco”, have been developed in order to meet these new requirements.
The improvement in the energy performance of lubricant compositions can be obtained in particular by mixing specific additives such as friction modifiers, polymers that improve the viscosity index, into base oils. Among the friction modifiers, the organometallic compounds comprising molybdenum are commonly used. In order to obtain good anti-friction properties, a sufficient quantity of molybdenum must be present within the lubricant composition. Among these organometallic compounds, molybdenum dithiocarbamates are most used as a source of molybdenum.
However, these compounds have the drawback of causing the formation of sediments when the lubricant composition has too high a content of elemental molybdenum. The poor solubility of these compounds modifies, or even degrades the properties of the lubricant composition, in particular its viscosity. Now, a composition which is too viscous or not viscous enough militates against the movement of the mobile parts, easy starting of an engine, the protection of an engine when it has reached its operating temperature, and therefore ultimately causes in particular an increase in fuel consumption.
Furthermore, these molybdenum dithiocarbamates contribute to increasing the ash content, reducing their potential for use in a lubricant composition, in particular in Europe. Moreover, the presence of friction modifiers in a lubricant composition can degrade the thermal resistance of the composition, and thus degrade the cleanliness of the engine.
Different technical solutions for replacing the compounds based on molybdenum have been described. Document EP 1 780 257 describes a lubricant composition comprising a polyglycerol ether, said composition having improved fuel economy properties. This document also describes the combination of this ether with a polymer of the polyisobutylene-succinimide type.
However, the polyglycerol ether described in this document has a linear structure. Furthermore, the lubricant composition described in this document finds its application on specific surfaces characterized by a low coefficient of friction, such as surfaces of the DLC (Diamond-Like Carbon) type. Moreover, no quantification of the fuel economy properties and no indication in respect of the thermal resistance of the lubricant composition are given in this document.
As fuel economy requirements are increasing, there is therefore still a need to seek novel friction modifiers which, once formulated in a lubricant composition, do not destabilize it, and allow improved fuel economy properties to be obtained. There is also the need to seek novel friction modifiers which, once formulated in a lubricant composition, do not destabilize it, and allow improved fuel economy properties to be obtained.
An objective of the present invention is to supply a friction modifier as well as a lubricant composition comprising said friction modifier that overcomes all or part of the aforementioned drawbacks. Another objective of the present invention is to supply a lubricant composition that is thermally stable and comprises very little or no compounds based on molybdenum. Another objective of the present invention is to supply a lubricant composition comprising very little or no compounds based on molybdenum and having equivalent, or even improved, friction reduction properties while being capable of application on different surfaces, in particular on surfaces of a different chemical nature. Another objective of the invention is to supply a lubricant composition the formulation of which is easy to implement. Another objective of the present invention is to supply a lubrication method allowing energy savings.
The purpose of the invention is thus a lubricant composition comprising:                at least one base oil, and        at least one polyglycerol ether of formula (I)        
in which:                R1 represents a linear or branched alkyl group, containing from 1 to 30 carbon atoms;        n represents an integer ranging from 2 to 10.        
In an embodiment of the invention, the lubricant composition can comprise:                at least one base oil,        at least one polyglycerol ether of formula (I)        
in which:                R1 represents a linear or branched alkyl group, containing from 1 to 30 carbon atoms;        n represents an integer ranging from 2 to 10, and        at least one dispersant.        
In another embodiment of the invention, the lubricant composition can comprise:                at least one base oil,        at least one polyglycerol ether of formula (I)        
in which:                R1 represents a linear or branched alkyl group, containing from 1 to 30 carbon atoms;        n represents an integer ranging from 2 to 10, and        at least one dispersant chosen from the compounds comprising at least one succinimide group, succinic acid esters or succinic acid amide esters.        
In another embodiment of the invention, the lubricant composition can comprise:                at least one base oil,        at least one polyglycerol ether of formula (I)        
in which:                R1 represents a linear or branched alkyl group, containing from 1 to 30 carbon atoms;        n represents an integer ranging from 2 to 10, and        at least one dispersant chosen from the compounds comprising at least one substituted succinimide group or the compounds comprising at least two substituted succinimide groups, the succinimide groups being linked at their vertex bearing a nitrogen atom by a polyamine group.        
Surprisingly, the applicant found that the presence of a polyglycerol ether of formula (I) in a lubricant composition makes it possible to improve the anti-friction properties of the composition, and thus to improve the fuel economy properties. Thus, the present invention makes it possible to formulate lubricant compositions comprising no, or very little, compounds based on molybdenum, and nevertheless having equivalent, or even improved, anti-friction and fuel economy properties.
Advantageously, the lubricant compositions according to the invention have an improved thermal stability. Advantageously, the lubricant compositions according to the invention have an improved stability on storage, as well as a viscosity that varies very little or not at all. Advantageously, the presence of at least one polyglycerol ether of formula (I) in a lubricant composition makes it possible to achieve fuel economies when a motor is idling or running at high speed.
In an embodiment, R1 represents a linear or branched alkyl group, containing from 8 to 25 carbon atoms, preferably from 10 to 20 carbon atoms. In an embodiment, n represents 2, 3, 4 or 5, preferably 2, 3 or 4.
In an embodiment, the polyglycerol ether is chosen from the compounds of formula (I) in which:                R1 represents a linear or branched alkyl group, containing 12 carbon atoms and n represents 2; or        R1 represents a linear or branched alkyl group, containing 18 carbon atoms and n represents 2; or        R1 represents a linear or branched alkyl group, containing 16 carbon atoms and n represents 3; or        R1 represents a linear or branched alkyl group, containing 12 carbon atoms and n represents 4; or        R1 represents a linear or branched alkyl group, containing 18 carbon atoms and n represents 4.        
In an embodiment, the lubricant composition consists essentially of at least one base oil and at least one polyglycerol ether of formula (I)
in which:                R1 represents a linear or branched alkyl group, containing from 1 to 30 carbon atoms;        n represents an integer ranging from 2 to 10.        
In an embodiment, the lubricant composition consists essentially of at least one base oil, at least one dispersant and at least one polyglycerol ether of formula (I)
in which:                R1 represents a linear or branched alkyl group, containing from 1 to 30 carbon atoms;        n represents an integer ranging from 2 to 10.        
In an embodiment, the dispersant is chosen from the compounds comprising at least one succinimide group, succinic acid esters or succinic acid amide esters. In an embodiment, the dispersant is chosen from the compounds comprising at least one substituted succinimide group or the compounds comprising at least two substituted succinimide groups, the succinimide groups being linked at their vertex bearing a nitrogen atom by a polyamine group.
In an embodiment, the dispersant is a substituted succinimide of formula (II) or a substituted succinimide of formula (III):

in which:                x represents an integer ranging from 0 to 10, preferably 2, 3, 4, 5 or 6;        y represents an integer ranging from 2 to 6, preferably 2, 3 or 4;        R2 represents an alkyl group comprising from 8 to 400 carbon atoms, preferably from 50 to 200 carbon atoms, an aryl group comprising from 8 to 400 carbon atoms, preferably from 50 to 200 carbon atoms, an arylalkyl group comprising from 8 to 400 carbon atoms, preferably from 50 to 200 carbon atoms or an alkylaryl group comprising from 8 to 400 carbon atoms, preferably from 50 to 200 carbon atoms;        R3 and R4, identical or different, represent independently a hydrogen atom, a linear or branched alkyl group comprising from 1 to 25 carbon atoms, an alkoxy group comprising from 1 to 12 carbon atoms, an alkylene group comprising from 2 to 6 carbon atoms, a hydroxylated alkylene group comprising from 2 to 12 carbon atoms or an alkylene amine group comprising from 2 to 12 carbon atoms.        
In an embodiment, the dispersant is a substituted succinimide of formula (II) in which:                R2 represents a polyisobutylene group,        R3 and R4 represent a hydrogen atom,        x represents 2,        y represents 2 or 3.        
In an embodiment, the polyglycerol ether content by weight ranges from 0.1 to 3%, preferably from 0.5 to 2%, with respect to the total weight of the lubricant composition. In an embodiment, the dispersant content by weight ranges from 0.1 to 10%, preferably from 0.1 to 5%, advantageously from 0.1 to 3% with respect to the total weight of the lubricant composition. In an embodiment, the mass ratio (mass of polyglycerol ether/mass of dispersant) ranges from 5/1 to 1/5, preferably from 2/1 to 1/2. In an embodiment, the lubricant composition comprises moreover at least one additive chosen from detergents, anti-wear additives, extreme-pressure additives, antioxidants, polymers improving the viscosity index, pour point improvers, anti-foaming agents, thickeners and mixtures thereof.
The invention also relates to an engine oil comprising a lubricant composition as defined above. It relates to the use of a lubricant composition as defined above for reducing the fuel consumption of vehicles. It relates to a method for reducing the energy losses by friction of a mechanical part comprising at least one step of bringing a mechanical part into contact with a lubricant composition as defined above. It relates to a method for reducing the fuel consumption of a vehicle comprising at least one step of bringing a mechanical part into contact with a lubricant composition as defined above.
It relates to the use of a polyglycerol ether of formula (I) as a friction modifier in a lubricant composition

in which:                R1 represents a linear or branched alkyl group, containing from 1 to 30 carbon atoms;        n represents an integer ranging from 2 to 10.        